1. Field of the Invention
The present invention relates to accurately determining the position of an object along a linear trajectory without making physical contact with the object, and more specifically, it relates to systems for determining the position of a magnetic levitation car relative to the track above which it moves.
2. Description of Related Art
There are numerous situations when it is required to use non-contacting means to measure the location along a linear track of a moving object. An example of such a situation is the problem of accurately locating the position and velocity of a magnetically levitated train car moving above a linear track. Accurate information on the location of the car in such cases would, for example, be needed in order to synchronize the drive currents of a Linear Synchronous Motor (LSM) drive with respect to the position of the train. In such drive systems the propulsion is obtained by exciting currents in multi-phased windings that are embedded in the track. These currents interact with the magnetic fields arising from an array of permanent magnets on the moving train car. In order to use the LSM drive system to accelerate the car, to keep it in motion at a constant speed, and then to decelerate it, the phase, amplitude and frequency of the currents in the LSM windings must be accurately controlled at all times. This requirement must be met by actively controlling the inverter that supplies the currents to the track. However to achieve this end it is necessary that the train car should be able to communicate its position, within an accuracy of a few millimeters, to the control circuits of the inverters. It is also required that the location method used should be insensitive to variations in the levitation height of the train car as might be caused, for example, by changes in the passenger loading.